Check craigslist for used stall mats. When I built my barn, I was able to get enough to do over one full stall for very cheap off of craigslist. Of course, I had to clean them which was a dirty job, but if you're on a limited budget it is certainly the way to go. Some factories will also give you their old large rubber conveyer belts for cheap or free-- I know someone who used these for a 20 stall barn. They are thinner though, but for only a few hours use per day, they should be more than fine.
Before I built my barn, my horses were at a barn with dirt floors, and installing the mats was a first priority for me before they came home-- sooooooo much cleaner and nicer!